This Is How An Automatic Transmission Works

Illustration for article titled This Is How An Automatic Transmission Works

Ever wonder how your transmission knows to shift gears? Why is it that when you stop, the engine doesn’t die? We’re here to show you how cars work. We recently looked at manual transmissions. This week it’s regular ol’ slushbox time.

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Automatic transmissions – they’re pretty much black magic. The sheer number of moving parts makes them very difficult to comprehend. Let’s simplify it a bit to get a basic understanding of how it all works in a traditional, torque converter-based system.

Your engine connects to your transmission at a place called a bell housing. The bell housing contains a torque converter for automatic transmission-equipped vehicles as opposed to a clutch on manual vehicles. The torque converter is a fluid coupling whose job it is to connect your engine to your transmission and thus to your driven wheels. The transmission contains planetary gearsets which are in charge of providing different gear ratios. To get a good understanding of how the whole automatic transmission system works, let’s have a look at torque converters and planetary gearsets.

Torque Converter

First and foremost, your engine’s flex plate (basically a flywheel for an automatic) connects directly to a torque converter. So when the crankshaft rotates, so does the torque converter housing. The goal of the torque converter is to provide a means by which to connect and disconnect the engine’s power to the driven load. The torque converter takes the place of a clutch on a conventional manual transmission. How does the torque converter work? Well, have a look at the video above. It explains the basic principles behind a fluid coupling. Once you’ve watched that, continue reading to see how a torque converter differs from a standard fluid coupling.

The major components of a torque converter are: the impeller, the turbine, the stator, and the lock-up clutch. The impeller is part of the torque converter housing, which is connected to the engine. It drives the turbine via viscous forces. The turbine is connected to the transmission input shaft. In essence, the engine turns the impeller which imparts forces on a fluid, which then rotates the turbine, sending torque to the transmission.

The transmission fluid flows in a loop between the impeller to the turbine. The fluid coupling in the video above suffers from severe churning losses (and consequent heat buildup) as the fluid returning from the turbine has a component of its velocity that opposes the rotation of the impeller. That is, the fluid returning from the turbine works against the impeller’s rotation and thus against the engine.